Keywords: biorobotics, plant roots, biomechatronic system, plantoid, osmotic actuator, swarm behaviorOur vision of plants is changing dramatically: from insensitive and static objects to complex living beings able to sense theenvironment and to use the information collected to adapt their behavior. At all times humans imitate ideas and conceptsfrom nature to resolve technological problems. Solutions coming from plants have the potential to face challenges anddifficulties of modern engineering design. Characteristic concepts of the plant world such as reiteration, modularity andswarm behavior could be of great help resolving technological problems. On the other hand a biorobotic approach wouldfacilitate the resolution of many biological problems. In this paper, the concept of a plant-inspired robot is proposed forthe investigation of both biological and technological issues.

Somewhere between 4001,000 million years ago, plants (and

some animals such as corals) initiated a sessile lifestyle, takingadvantage of the ubiquity of light as a source of energy as well asdevising ways of compensating for body losses suffered because ofbrowsing predators. Among the primary advances made by plantsand sessile animals to survive predation was the evolution of different modular structures.1 Roots, leaves, branches, shoots, buds,flowers, are reiterated many times during the development of asingle plant body, to ensure that in case of environmental damageor predation some module of the body can survive and regeneratethe individual. In general, as a consequence of this primordialdecision for a sessile and modular lifestyle, the specialization oftissues and cells in plants is minimized, if compared with animals, to limit predatory damages.Another consequence of the sessile decision was the need ofa well-organized sensing system which allows plants to explore efficiently the environment and to react rapidly to potential dangerous circumstances. Below ground, roots can sense a multitude ofabiotic and biotic signals, providing all the time the appropriateresponses. Actually, roots behave almost like active animals,2,3performing efficient exploratory movements, with the root apicesthat drive the root growth in search for air, nutrients and waterto feed the whole plant body. Interestingly, modularity, reiteration and evolved sensing systems are among the most importantproblems of today robotics.The perspective we are looking at plants in the last years ischanging dramatically, tending away from seeing them as passive entities subject to environmental forces and organisms thatare designed solely for accumulation of photosynthetic products.The new vision, by contrast, is that plants are dynamic and highlysensitive organisms, with complex behaviors4-6 actively and competitively foraging for limited resources both above and belowground. They are also organisms, which accurately compute theircircumstances, use sophisticated cost-benefit analysis and take

defined actions to mitigate and control diverse environmental

insults.7-9 Therefore, plants can be considered as informationprocessing organisms with complex communication throughout the individual body. In addition, the architecture of theirbody and their physiological attitudes make plants an unlimitedsource of inspiration for robotic scientists. In the next paragraphswe will discuss the possibility that plants could be very usefulfor robotic studies with return for both robotic and biologicalsciences.BioroboticsBiorobotics is a new scientific and technological area with aunique interdisciplinary character, aimed at increasing knowledge on how biological systems work. This objective may beobtained by (1) analyzing living organisms from a biomechatronicperspective and (2) exploiting the obtained knowledge to developinnovative methodologies and technologies (Fig. 1). Bioroboticsencompasses the dual use of a biorobot as a tool for biologistsstudying living organisms behavior and as test-bed in the studyand evaluation of biological models for potential applications inengineering. As a result, the interaction between biological scienceand robotics becomes two-fold: on one hand, biology providesthe knowledge of the biological systems needed to build biorobots, on the other hand, bio-inspired robots represent a helpfulplatform for experimental validation of theories and hypothesesformulated by scientists.10In plant science, biorobots can be a valuable tool when theplant is studied as a system. Hypotheses are formulated on theoverall working principle of the plant, including the interrelationsamong the different organs. In these studies the use of a biorobot may result advantageous, being programmable and reconfigurable to test different models and enabling plant scientiststo study the complex biological processes occurring in plants in

Plant Signaling & Behavior

Figure 1. A schematic view of the loop for the formulation of a formal model of a biological system, by experimental validation.

their wholeness, adapting design-based engineering principles

to biological systems.Robotics and biology can thus combine together in a commonresearch program, which leads to better scientific understandingof plants, animals and humans. Figure 2 shows the loop froman hypothesized formal model of the biological system, to theexperimental implementation and test, and then to the formal

revision, that ultimately leads to the validation of the formal

model as a satisfactory explanation of the biological system. Thisscheme allows one to ideally establish the roles of roboticists andbiologists in the explanation path. Ideally, (1) biologists hypothesize the functional formal model, (2) roboticists implement themodel; (3) a joint collaboration of roboticists and biologists isrequired during the experimental monitoring phase and finally(4) biologists do the revision.The PlantoidA Plant-Inspired Robot

Figure 2. Conceptual scheme of the biomechatronics scientific/technological paradigm: a typical mechatronic system is characterized bythe smooth and effective integration of its fundamental components(mechanisms, sensors, control, actuators, power supply), and by the factthat such integration is included in the components and system designprocess, from the very beginnings; biomechatronics considers thesystem together with its interactions with the external world and withthe human operator, which become a source of biological inspiration,on one side, and a reference for functional specifications, in systems forbiomedical applications, on the other.

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Plants represent an excellent source of inspiration for developing a

new generation of technologies, for example robots. Traditionally,robotics look to the animals for drawing inspiration in designing and developing new classes of biologically inspired robots.We think that the plant world could represent a real source ofinspiration in robotics as well. Plant specific behaviors, sensingand communication capabilities, processing, control and energystorage systems, make these organisms unique and excellentexamples to imitate in developing innovative and high-technology solutions.By inspiring to the plant life a robotic system would be ableto explore the above and belowground environment, acquiringinformation about vital parameters. A plantoid artefact withbranching sensory roots would explore the soil in a more efficient way by taking inspiration by and implementing the amazing movement and growing features of plant roots. A plantoidrobot would include root and shoot systems being able to changeits geometrical configuration and size according to the environmental conditions.Innumerable would be the practical applications of a plantinspired robotic artefacts: in situ monitoring analysis and chemical detections, water searching, anchoring capabilities, as wellas in developing new communication strategies or processing,and control algorithms inspired by plants. Last but not least, aplantoid would represent an excellent tool for the scientific studyof the plant behavior by building physical models. This new

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Figure 3. The root systems of different prairie plants. Modified from United States Department of AgricultureIllinois native plant guide.

robot generation can be considered as larger systems whose overall functionalities are designed to study the complex biologicalprocesses occurring in plants in their entirety, adapting designbased engineering principles to biological systems: the key principles of synthetic biology and biorobotics.Why bother building robots instead of just using computer/numerical models? The answer to this question comes fromthe complexity of the sensory world represented by plants andliving organisms in general. A hypothesis implemented on arobot operating in a real environment can be tested more rigorously and in a much more rational and systematic way thanin simulation because the hypothesis will be challenged withreal, complex and often unmodelable stimuli. Moreover, it ispossible to obtain orders of magnitude more data from a robot,compared to a plant, on its actions, its sensory input, and itsinternal states.A typical biomechatronic system (Fig. 2) can be considered to be composed of a mechanical part (usually an articulatedstructure with many degrees of freedom); of a number of proprioand extero-ceptive sensors; of actuators; of energy sources; of anetwork of microprocessors (usually embedded) and of analogand digital signal processing boards; of control interfaces and ofcommunication units.Following the biomechatronic structure, even the plantoidcan be divided in three main sub-systems: (1) a main body,carrying batteries, electronics and radio systems; (2) the rootsystem, with electro-osmotic actuators; (3) the root apex, withsensors. Robot leaves would include photovoltaic cells, toassure an energy power proper for carrying out operative functionalities.11 The plantoid roots will be able to grow accordingto the different stimuli, such as gravity and water or chemical gradients (possibly, different sensors can be included in theroot). The system will include actuators for the steering of theapices according to the data coming from the sensory apparatus. Each apex will embed a microcontroller module for theemulation of the roots behavior through the local implementation of models.12Taking inspiration by plants, new communication, coordination and interaction strategies can be designed and developed.Communication in plant could be only preliminary seen as atransposition of swarm-like intelligence to: (a) a single connected

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organism consisting of a high number of heterogeneous,

low-profiled and multifunctional parts; and (b) different organisms of this kind which can interact.Reiteration and Swarm Behavior of RootsMany animals when acting collectively show remarkable groupbehaviors. Swarming insects or flocking birds, by changing shapeand direction appear to move as a single coherent organism.This kind of aggregate motion, known as swarm behaviour, isstudied by biologists that want to understand how social animalsinteract or by computer scientists that try to apply the resulting swarm intelligence to optimize problems in fields as, forexample, telecommunication, robotic, and transportation systems.13-15,28 The advantage of using such techniques is double asthey produce robust behaviors without they use of a centralizedsynchronization center and with very simple communication protocols among single agents. In fact, computer simulations demonstrated that both central coordination and global informationare not necessary for collective behavior.16-18 Recently, Couzin19developed a robust model describing digitally the behavior offish schools by identifying a minimum amount of information(repulsion, attraction, heading alignment).If we look at the growing and explorative behavior of rootsin the soil, we cannot escape to notice that despite the lackingof a central nervous system and with few evidences of communication among root apices,29 the growth pattern of theroot apparatus is not chaotic at all. On the contrary it lookscoordinated and efficiently shaped to exploit soil resources(Fig. 3) and to avoid hazards. In addition, considering a plant asa colony of modular parts is not a new idea as already the Greekphilosopher Theophrastus wrote that repetition is the essenceof a plant. In the eighteenth century botanists such as Bradley,von Goethe26 and Erasmus Darwin20 thought that trees couldbe regarded as a colony of repeating parts. More recently, plantshave been described as metameric organisms21 i.e., their body iscomposed by a collection of unitary parts. Although reiteratedelements have mostly been considered as leafy branch systems,Hall22 posed the premise that these units would include also theroot system. Effectively, because of the recursive formulation andhierarchical levels found in plant root systems, the study and the

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simulation of the root growth have been frequently based on the

fractal analysis.23,24Plant roots exhibit an exceptional capacity to sense very weakoxygen, water, temperature and nutrient gradients in the soil.25However, how they manage to navigate toward the source ofthe resource without being distracted by local variations, remainunclear. An interesting analogy can be found in the collectivebehavior adopted by birds during a long-range migration. The navigation following a very weak gradient, due to local variations, it isan almost impossible task for an individual bird, whereas a flockacting collectively overtake this obstacle by working as an integrated array of sensors.27 Consequently, it is tempting to proposethat a collective behavior of root tips could emerge even in plantroots from the individual activity of the single root apex. In thesame way of birds, for example, individual root apex could adopta collective behaviour that minimize the influence of local fluctuations, for exploration purposes. The swarm rules come outas simple rules for the different organisms: information left by thechemical traces produced by every single apex and instruction oflocal density may be considered the basis of such rules for roots.ConclusionsIn conclusion, robotic artefacts will reproduce a plant-likestructure and the system will be designed as a distributed architecture both from the hardware and the software viewpoint. Theplantoid will consist of many modules, which will represent theirReferences1.

natural counterparts structure and functionalities, namely, leaves,

branches, stem, roots, apexes. Each module will be computationally low-profiled and task-specialized (as a root apex for instance),with a high degree of autonomy, integrating its own processingunit with a stored set of basic behavioral rules that in general willdepend from the specific module. The idea leads also to conceptsof robustness and flexibility of the system: a plantoid composedof several distributed and self-organized modules would grant ahigher probability of survival compared to a centralized systemdisposing of few functionalized parts, as well known from multiagent or swarm intelligence theory.This functionality may be used in solving such problems ase.g., energy harvesting and management, supporting of internalhomeostasis, reconfiguration, sensor fusion, collective environmental awareness and others. On the basis of its own strategiesand behavioural rules each functional module in the plantoidwill communicate with all the other units composing the robot,making the whole plantoid a complex and highly-evolved system where decisions emerges in the form of the best compromisebetween a large amount of independent and integrated internalstimuli and requests.The outcome of such a plantoid would not be limited todevelop emergent decision strategies for a complex robotic systemaimed at autonomously monitoring environment for extendedperiods of time, but it would eventually allow an invaluable feedback on the scientific knowledge of control and communicationsstrategies intra- and inter-plants.